1. Field of the Invention
The present invention relates to an apparatus for and a method of detecting a defocus error signal for an optical pickup and an apparatus for and a method of detecting a seek direction of a recording medium having a land/groove structure adopting the same.
2. Description of the Related Art
For a recording medium of a next generation DVD family, that is, a recording medium requiring higher density recording than a recording medium of a DVD family, where information is recorded by being defocused, an effect of defocusing is much greater than other existing recording media which use a red laser beam. Thus, a method of compensating for defocusing is needed.
To accommodate high density recording, where a short wavelength light within a blue wavelength range (for example, 420 nm or less) and an objective lens having a numerical aperture (NA) greater than 0.6 are used, the depth of focus of an incident beam is shallow so that a margin for defocus is reduced. Accordingly, a small amount of defocus may be a problem in recording. For a recording medium of the next generation DVD (a so-called HD-DVD) family, a light source for emitting light having a wavelength shorter than a red wavelength and an objective lens having an NA of 0.6 or more are expected to be used.
FIG. 1 shows a change in intensity of a light spot formed on a recording medium according to defocus. In FIG. 1, the horizontal axis denotes defocus and the vertical axis denotes normalized beam peak intensity. As defocus increases, the intensity of an incident beam having a 400 nm wavelength and an 0.65 NA sharply decreases relative to the intensity of an incident beam having a 650 nm wavelength and an 0.60 NA. Where recording is performed under these conditions, a mark having a desired length and width cannot be recorded. For incident beams having the same wavelength, as the NA increases, the intensity an incident beam is reduced according to defocus.
FIG. 2 shows a change in size of a light spot formed on a recording medium according to defocus. In FIG. 2, the horizontal axis denotes defocus and the vertical axis denotes light spot size ratio. As defocus increases, the spot size of a beam having a 400 nm wavelength increases more rapidly than a spot size of a beam having a 650 nm wavelength. Although the light beams have the same wavelength, as the NA increases, the spot size increases.
As well known in the field to which the present invention pertains, where a recording medium in a DVD family is recorded by using an objective lens having a 0.6 NA and a light beam having a 650 nm wavelength, control of defocus of about 230 nm is needed. However, for example, where a recording medium of a next generation DVD family is recorded by using a blue beam and an objective lens having a 0.85 NA, defocus should be controlled within tens of nanometers.
Thus, to finely control defocus within tens of nanometers where recording is performed with respect to the next generation DVD family, a defocus error signal, which is not affected by a tilt of a recording medium relative to an objective lens, detrack, and a shift of the objective lens needs to be detected.